The present invention relates generally to medical devices, and more particularly to a method and apparatus for forming a three-dimensional wireform for use in heart valves.
Two primary types of heart valve replacements or prostheses are known. One is a mechanical-type heart valve that uses a ball and cage arrangement or a pivoting mechanical closure to provide unidirectional blood flow. The other is a tissue-type or xe2x80x9cbioprostheticxe2x80x9d valve which is constructed with natural-tissue valve leaflets which function much like those in a natural human heart valve, imitating the natural action of the flexible leaflets that form commissures to seal against each other and ensure the one-way blood flow. In tissue valves, a whole xenograft valve (e.g., porcine) or a plurality of xenograft leaflets (e.g., bovine pericardium) provide occluding surfaces that are mounted within a surrounding stent structure.
In both types of bioprosthetic valves, an elastic wireform, typically metal, provides internal valve support for the flexible leaflets. The wireform has a three-dimensional shape, with a plurality (typically three) of large radius cusps supporting the cusp region of the leaflets of the bioprosthetic tissue (i.e., either a whole valve or three separate leaflets). The free ends of each two adjacent cusps converge somewhat asymptotically to form upstanding commissures that terminate in tips, each being curved in the opposite direction as the cusps and having a relatively smaller radius.
The wireform is typically a non-ferromagnetic metal such as Elgiloy that possesses substantial elasticity. A common method of forming the wireform is to bend a wire into a flat (2-dimensional) undulating pattern of the alternating cusps and commissures, and then roll the flat pattern into a cylinder using a cylindrical roller. The free ends of the resulting 3-dimensional shape, typically in the asymptotic region of the cusps, are then fastened together using a tubular crimp. Because of the configuration (i.e., curvatures, circumferential arcs, axial lengths) of the alternating cusps and commissures, the elasticity (springback) of the material, and the required overall diameter of the 3-dimensional wireform, the free ends do not meet after the rolling step. Rather, there is some separation, and the wire must be constricted to a degree to bring the free ends together for crimping. As a result, the 3-dimensional wireform exhibits an outward spring bias in its final form, which may affect the dynamic response and fatigue life in the finished valve, and thus must be factored into the design. This additional design consideration is undesirable, and the performance of the valve may suffer from the wireform bias.
What is needed then is an improved method and apparatus for forming an unbiased three-dimensional heart valve wireform.
The present invention provides an apparatus for forming a three-dimensional wireform for use in a heart valve, comprising a first mandrel having a forming surface with a varying curvature, and a second mandrel having a forming surface with a varying curvature. The apparatus further includes a forming tool having clamps for cooperating with the mandrels and rigidly holding a two-dimensional wire and displacing the forming surfaces of the mandrels with respect to one another to bend the wire into a three-dimensional shape. The forming tool may include first and second shafts arranged to rotate along parallel axes, with the mandrels having throughbores sized to receive the respective shafts, wherein rotation of the shafts causes the displacement of the forming surfaces of the mandrels. The forming tool may include a linear slide for translating one of the shafts in a direction perpendicular to the parallel axes, such that displacement of the forming surfaces of the mandrels includes translating one of the shafts and forming surface toward the other shaft and forming surface. Motors coordinated by a programmable controller desirably rotate the shafts and translating the one shaft.
In one embodiment, the two dimensional wire includes alternating arcuate cusps and commissure tips, and the clamps and mandrels cooperate to hold two of the commissure tips with one of the arcuate cusps therebetween, the forming surfaces therefore bending the arcuate cusp of the wire when displaced. Each clamp and mandrel may be disposed on a shaft, with either the mandrel or clamp arranged to slide along the shaft with respect to the other to alternately hold and release the wire between the clamp and mandrel. The two dimensional wire desirably includes alternating arcuate cusps and commissure tips, and each mandrel includes a channel and cross-bores opening to the channel. Further, each clamp may have a recess aligned with the channel, wherein a pair of pins are provided fitting in the crossbores and sized so that one of the commissure tips may be wrapped therearound, the pins projecting into the channel a distance that permits sliding of the mandrel into cooperation with the clamp such that the projections of the pins fit within the clamp recess and the commissure tip is retained between the channel and recess.
Another aspect of the invention is a subassembly for bending a wire for a heart valve wireform, the wireform having alternating arcuate cusps and arcuate commissure tips. The subassembly includes a pair of mandrels having forming surfaces that have non-cylindrical cross-sectional shapes. Each mandrel may include a throughbore and a keyway, and the subassembly further includes, for each mandrel, a shaft sized to fit within the throughbore having an axial groove, and a key sized to fit within the groove in the shaft and within the keyway and couple rotation of the shaft and mandrel. The subassembly may further include, for each mandrel, a clamp having a throughbore and a keyway sized to receive the shaft and key, respectively, and couple rotation of the shaft, mandrel and clamp. Each mandrel preferably has an axial channel on one side adjacent the forming surface, and the clamp has an external recess aligned with the channel, the subassembly further including a retaining pin projecting into the channel and sized to receive therearound one of the arcuate commissure tips of the two-dimensional wire, the commissure tip being retained by overlapping of the channel and recess.
A method for forming a three-dimensional wireform for use in a heart valve is also provided by the present invention. The method comprises:
bending a wire into a flat (2-dimensional) undulating pattern of the alternating cusps and commissures; and
bending the flat pattern into a three dimensional shape using:
a pair of mandrels having forming surfaces that have non-cylindrical cross-sectional shapes.
The method may be accomplished using a forming tool having clamps for cooperating with the mandrels and rigidly holding a two-dimensional wire, wherein the method includes displacing the forming surfaces of the mandrels with respect to one another to bend the wire into a three-dimensional shape. The forming tool desirably has first and second shafts arranged to rotate along parallel axes, and the mandrels have throughbores sized to receive the respective shafts, wherein the method further includes rotating the shafts to cause the displacement of the forming surfaces of the mandrels. Additionally, the forming tool preferably includes a linear slide for translating one of the shafts in a direction perpendicular to the parallel axes, and the displacement of the forming surfaces of the mandrels further includes translating one of the shafts and forming surface toward the other shaft and forming surface. Rotation of the shafts and translation of the one shaft is desirably carried out using motors and a programmable controller. The method desirably involves holding two of the commissures of the wire with a cusp therebetween and using the pair of mandrels to bend one cusp at a time.